Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. The early combustion due to pre-ignition can cause high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity.
One example approach for identifying pre-ignition is illustrated by Inada et al. in U.S. Pat. No. 7,673,614. Therein, cylinder pre-ignition is detected based on ion currents and leakage currents across the spark plug. The method separately detects spark plug smoldering, again based on ion currents and leakage currents across the spark plug. Specifically, different profiles of the ion current/leakage current are used to identify either pre-ignition or spark plug smoldering.
However, the inventors herein have recognized that in some instances, pre-ignition is caused by a degraded spark plug. As such, the above approach can potentially provide inaccurate results indicating that pre-ignition has occurred (e.g., due to excessive boost), whereas the true culprit is a degraded spark plug. Additionally, the inaccurate results can lead to inaccurate mitigating steps being taken, that can potentially damage the engine.
In one example, some of the above issues may be addressed by a method of controlling an engine cylinder including a spark plug. One example embodiment comprises, indicating spark plug degradation in a cylinder based on each of misfire and pre-ignition in the cylinder. For example, the indication of degradation may be based on an occurrence of misfire in the cylinder and an occurrence of a number of pre-ignition events in that cylinder occurring within a threshold duration.
In this way, it is possible to distinguish pre-ignition due to excessive boost or compression ratio, from pre-ignition due to spark plug degradation. Specifically, by correlating the cylinder's misfire history with the cylinder's pre-ignition history, it is possible to reliably identify pre-ignition induced by spark plug degradation and mitigating actions can be taken in accordance. For example, by limiting combustion in a cylinder having a degraded spark plug, especially at high engine loads, further cylinder pre-ignition events may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.